1. Field of the Invention and Contract Statement
The invention relates to primary containment media for the disposal of high-level radioactive nuclear waste. The United States Government has rights in this invention pursuant to Contract No. DE-W-7405-eng-26 between the U.S. Department of Energy and Union Carbide Corporation.
2. Discussion of Background and Prior Art
In the past, nuclear waste has been temporarily stored, frequently as a liquid or as a sludge in conjunction with a liquid. The art has recognized that means must be provided for permanent disposal of the waste, preferably as highly stable solids. Such solids must have certain characteristics which make such solids safe and economical for the long-term (10.sup.3 to 10.sup.5 years) retention of radioactive waste isotopes.
Because of the long half-lives of some radionuclides (e.g., certain actinide isotopes), it is necessary that the selected storage medium exhibit certain properties in order to achieve the desired long-term stability. Some of the factors which must be considered in the selection of a storage medium include: high chemical stability, i.e., low corrosion rates; structural stability; simple to manufacture; acceptable preparation temperature; ability to store a high proportion of waste to insure minimum storage volume; and availability to components making up the storage medium.
Various glass compositions have been suggested and tested for suitability as a storage medium. The borosilicate glasses have been considered among the more promising compositions. However, the borosilicate glasses have demonstrated significant instability under hydrothermal conditions, i.e., exposure to water at temperatures greater than 100.degree. C. Such hydrothermal conditions can be encountered in deep geological repositories.
Two highly desirable properties of any potential nuclear waste glass are a low preparation temperature and a low melt viscosity at the glass processing temperature. Pure lead phosphate glasses exhibit both of these properties [see: Argyle, J. F., and F. A. Hummel, J. Amer. Ceram. Soc. 43 (1960) 452; Osterheld, R. K. and R. P. Langguth, J. Amer. Chem. Soc. 59 (1955) 76; Ray, N. H., Glass Tech. 16 (1975) 107; Klonkowski, A., Phys. and Chem. Glasses 22 (1981) 163; and Furdanowicz, H. and L. C. Klein, Glass Tech. 24 (1983) 198]. Unfortunately, it is well known that these substances are susceptable to aqueous corrosion and that they tend to devitrify at temperatures as low as 300.degree. C. [see: Furdanowicz, H., et al., ibid.; Ray, N. H., C.J. Lewis, J.N.C. Laycock and W.D. Robinson, Glass Tech. 14 (1973) 50; and Longman, G.W., and G. D. Wignall, J. Mat. Sci. 8 (1973) 212].
Scientific Basis for Nuclear Waste Management, Vol. 1, Edited by G. J. McCarthy, Plenum Press (1979), pp. 43-50, 69 to 81 and 195 to 200. The phosphate glasses described in the reference include sodium aluminum phosphate glasses, or very complicated combinations of metal oxides and P.sub.2 O.sub.5. Of those phosphate glasses in the reference, only p. 74, Table 2, shows a composition containing lead oxide along with phosphorus pentoxide and nuclear waste oxides. The ratio of the phosphorus to lead content is very high and the phosphate glasses discussed therein are a multicomponent mixture of up to eight oxides.
In addition, the composition ranges given for each oxide in the reference on page 74 covers such a broad spectrum of possible phosphate glasses that the table has no significance due to the lack of specificity resulting from an effectively infinite array of permutations and combinations of glass constituents and concentrations.
See also: Scientific Basis for Nuclear Waste Management, Vol. 2, Edited by C. J. M. Northrup, Jr., Plenum Press (1980), p. 109 to 116; Report BNL-50130, Development of the Phosphate Glass Process for Ultimate Disposal of High-Level Radioactive Waste, R. F. Drager, et al., January 1968; and Symposium on Management of Radioactive Wastes from Fuel Reprocessing, Nov. 27 to Dec. 1, 1972, pp. 593-612.
No non-patent reference was found that indicated that lead-iron phosphate glasses have ever been seriously considered as a viable potential storage medium for the immobilization of nuclear wastes.
The glass and ceramic fields include the following domestic patents.
U.S. Pat. No. 3,365,578 (Grover et al.) discloses placing radioactive waste in a Na-Pb-Fe-phosphate/silicate glass, within a steel vessel. (Other Na-Pb-phosphate systems are disclosed in the examples of Grover et al.) To recap, Grover et al. teaches the use of a glass containing both Pb and phosphate for nuclear waste containment.
U.S. Pat. No. 4,314,909 (Beall et al.) teaches glass-ceramic which is used for waste storage and which consists of monazite, pollucite and ZrO.sub.2 and/or mullite. The glass-ceramic can contain up to 20 percent of P.sub.2 O.sub.5. Beall et al. does not mention the presence of Pb.
U.S. Pat. No. 4,351,749 (Ropp I) teaches nuclear waste storage blocks which include a polymeric phosphate glass from a trivalent metal selected from Al, In or Ga.
U.S. Pat. No. 4,382,974 (Yannopoulos) discloses a glass containing nuclear waste which is stabilized by the application of synthetic monazite by means of chemical vapor deposition or detonation gun. The monazite contains 27 to 35 weight percent of P.sub.2 O.sub.5. No Pb is mentioned in Yannopoulos.
U.S. Pat. No. 3,161,600 (Barton I) and U.S. Pat. No. 3,161,601 (Barton II), respectively, show Sr and Cs sequestrated in phosphate glasses.
U.S. Pat. No. 3,120,493 (Clark et al.) teaches a process wherein ruthenium volatilization is suppressed during the evaporation and calcination of nuclear waste solutions by the addition of phosphite or hypophosphite. A glass-like solid is obtained.
U.S. Pat. No. 4,049,779 (Ropp II) teaches stable phosphate glasses of formula M(H.sub.2 PO.sub.4)n, wherein M may be Pb and n is 2 or 3 (for divalent or trivalent M), which are prepared via H.sub.3 PO.sub.4 and a metal compound by adding a precipitant, crystallizing from solution and then melting the material. While Ropp II discloses lead phosphate glasses, it is not directed to nuclear waste disposal, although it does not mention stability to leaching.
U.S. Pat. No. 3,994,823 (Ainger et al.) discloses lead zirconate ceramic, which may also contain Bi. U may be added to reduce electrical resistivity. The ceramic of Ainger et al. is not aimed at nuclear waste storage.